Atrial arrhythmia episode detection in a cardiac medical device

ABSTRACT

A method and medical device for determining a cardiac episode that includes sensing a cardiac signal, identifying the signal sensed during a predetermined time interval as one of a cardiac event, a non-cardiac event, and an unclassified event, determining a number of identified cardiac events, determining a number of identified unclassified events, and determining whether the cardiac episode is occurring in response to the number of identified cardiac events being greater than a cardiac event count threshold and the number of identified unclassified events being less than an unclassified event count threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/520,938, entitled “ATRIAL ARRHYTHMIA EPISODE DETECTION IN A CARDIACMEDICAL DEVICE,” filed Oct. 22, 2014, issued as U.S. Pat. No. 10,219,718on Mar. 5, 2019, the content of which is incorporated by reference inits entirety.

TECHNICAL FIELD

The disclosure relates generally to cardiac medical devices and, inparticular, to methods for detecting atrial arrhythmia episodes duringventricular pacing in a cardiac medical device.

BACKGROUND

During normal sinus rhythm (NSR), the heart beat is regulated byelectrical signals produced by the sino-atrial (SA) node located in theright atrial wall. Each atrial depolarization signal produced by the SAnode spreads across the atria, causing the depolarization andcontraction of the atria, and arrives at the atrioventricular (A-V)node. The A-V node responds by propagating a ventricular depolarizationsignal through the bundle of His of the ventricular septum andthereafter to the bundle branches and the Purkinje muscle fibers of theright and left ventricles.

Atrial tachyarrhythmia includes the disorganized form of atrialfibrillation and varying degrees of organized atrial tachycardia,including atrial flutter. Atrial fibrillation (AF) occurs because ofmultiple focal triggers in the atrium or because of changes in thesubstrate of the atrium causing heterogeneities in conduction throughdifferent regions of the atria. The ectopic triggers can originateanywhere in the left or right atrium or pulmonary veins. The AV nodewill be bombarded by frequent and irregular atrial activations but willonly conduct a depolarization signal when the AV node is not refractory.The ventricular cycle lengths will be irregular and will depend on thedifferent states of refractoriness of the AV-node.

As more serious consequences of persistent atrial arrhythmias have cometo be understood, such as an associated risk of relatively more seriousventricular arrhythmias and stroke, there is a growing interest inmonitoring and treating atrial arrhythmias.

Methods for discriminating arrhythmias that are atrial in origin fromarrhythmias originating in the ventricles have been developed for use indual chamber implantable devices wherein both an atrial EGM signal and aventricular EGM signal are available. Discrimination of arrhythmias canrely on event intervals (PP intervals and RR intervals), event patterns,and EGM morphology. Such methods have been shown to reliablydiscriminate ventricular arrhythmias from supra-ventricular arrhythmias.In addition, such methods have been developed for use in single chamberimplantable devices, subcutaneous implantable devices, and externalmonitoring devices, where an adequate atrial EGM signal havingacceptable signal-to-noise ratio is not always available for use indetecting and discriminating atrial arrhythmias. However, such singlechamber devices have been designed to monitor AF during non-pacedventricular rhythm. What is needed, therefore, is a method formonitoring atrial arrhythmias during a ventricular paced rhythm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary medical device fordetecting arrhythmia during ventricular pacing according to anembodiment of the present disclosure.

FIG. 2 is a functional block diagram of an IMD according to oneembodiment.

FIG. 3 is flowchart of a method for detecting atrial arrhythmias duringventricular pacing in a cardiac medical device according to anembodiment of the present disclosure.

FIG. 4 is a schematic diagram illustrating detecting atrial arrhythmiasduring ventricular pacing in a cardiac medical device according to anembodiment of the present disclosure.

FIG. 5 is a schematic diagram illustrating detecting atrial arrhythmiasduring ventricular pacing in a cardiac medical device according toanother embodiment of the present disclosure.

FIG. 6 is a schematic diagram of classifying of cardiac events in acardiac medical device according to an embodiment of the presentdisclosure.

FIG. 7 is a diagram of an exemplary two-dimensional histogramrepresenting a Lorenz plot area for identifying cardiac events.

FIG. 8 is a flowchart of classification of an arrhythmia according to anembodiment of the disclosure.

FIG. 9 is a schematic diagram of determination of a cardiac event,according to the present disclosure.

FIG. 10 is a flowchart of determination of a cardiac episode, accordingto an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments for carrying out the methods described herein. It isunderstood that other embodiments may be utilized without departing fromthe scope of the disclosure.

In various embodiments, ventricular signals are used for determiningsuccessive ventricular cycle lengths for use in detecting atrialarrhythmias. The atrial arrhythmia detection methods do not require anatrial signal source. The methods presented herein may be embodied insoftware, hardware or firmware in implantable or external medicaldevices. Such devices include implantable monitoring devices havingcardiac EGM/ECG monitoring capabilities and associated EGM/ECG senseelectrodes, which may be intracardiac, epicardial, or subcutaneouselectrodes.

The methods described herein can also be incorporated in implantablemedical devices having therapy delivery capabilities, such as singlechamber or bi-ventricular pacing systems or ICDs that sense the R-wavesin the ventricles and deliver an electrical stimulation therapy to theventricles. The atrial arrhythmia detection methods presently disclosedmay also be incorporated in external monitors having ECG electrodescoupled to the patient's skin to detect R-waves, e.g. Holter monitors,or within computerized systems that analyze pre-recorded ECG or EGMdata. Embodiments may further be implemented in a patient monitoringsystem, such as a centralized computer system which processes data sentto it by implantable or wearable monitoring devices.

FIG. 1 is a schematic diagram of an exemplary medical device fordetecting arrhythmia during ventricular pacing according to anembodiment of the present disclosure. As illustrated in FIG. 1, amedical device according to an embodiment of the present disclosure maybe in the form of an implantable cardioverter defibrillator (ICD) 10 aconnector block 12 that receives the proximal ends of a rightventricular lead 16, a right atrial lead 15 and a coronary sinus lead 6,used for positioning electrodes for sensing and stimulation in three orfour heart chambers. Right ventricular lead 16 is positioned such thatits distal end is in the right ventricle for sensing right ventricularcardiac signals and delivering pacing or shocking pulses in the rightventricle. For these purposes, right ventricular lead 16 is equippedwith a ring electrode 24, an extendable helix electrode 26 mountedretractably within an electrode head 28, and a coil electrode 20, eachof which are connected to an insulated conductor within the body of lead16. The proximal end of the insulated conductors are coupled tocorresponding connectors carried by bifurcated connector 14 at theproximal end of lead 16 for providing electrical connection to the ICD10. It is understood that although the device illustrated in FIG. 1 is adual chamber device, other devices such as single chamber devices may beutilized to perform the technique of the present disclosure describedherein.

The right atrial lead 15 is positioned such that its distal end is inthe vicinity of the right atrium and the superior vena cava. Lead 15 isequipped with a ring electrode 21 and an extendable helix electrode 17,mounted retractably within electrode head 19, for sensing and pacing inthe right atrium. Lead 15 is further equipped with a coil electrode 23for delivering high-energy shock therapy. The ring electrode 21, thehelix electrode 17 and the coil electrode 23 are each connected to aninsulated conductor with the body of the right atrial lead 15. Eachinsulated conductor is coupled at its proximal end to a connectorcarried by bifurcated connector 13.

The coronary sinus lead 6 is advanced within the vasculature of the leftside of the heart via the coronary sinus and great cardiac vein. Thecoronary sinus lead 6 is shown in the embodiment of FIG. 1 as having adefibrillation coil electrode 8 that may be used in combination witheither the coil electrode 20 or the coil electrode 23 for deliveringelectrical shocks for cardioversion and defibrillation therapies. Inother embodiments, coronary sinus lead 6 may also be equipped with adistal tip electrode and ring electrode for pacing and sensing functionsin the left chambers of the heart. The coil electrode 8 is coupled to aninsulated conductor within the body of lead 6, which provides connectionto the proximal connector 4.

The electrodes 17 and 21 or 24 and 26 may be used as true bipolar pairs,commonly referred to as a “tip-to-ring” configuration. Further,electrode 17 and coil electrode 20 or electrode 24 and coil electrode 23may be used as integrated bipolar pairs, commonly referred to as a“tip-to-coil” configuration. In accordance with the invention, ICD 10may, for example, adjust the electrode configuration from a tip-to-ringconfiguration, e.g., true bipolar sensing, to a tip-to-coilconfiguration, e.g., integrated bipolar sensing, upon detection ofoversensing in order to reduce the likelihood of future oversensing. Inother words, the electrode polarities can be reselected in response todetection of oversensing in an effort to reduce susceptibility ofoversensing. In some cases, electrodes 17, 21, 24, and 26 may be usedindividually in a unipolar configuration with the device housing 11serving as the indifferent electrode, commonly referred to as the “can”or “case” electrode.

The device housing 11 may also serve as a subcutaneous defibrillationelectrode in combination with one or more of the defibrillation coilelectrodes 8, 20 or 23 for defibrillation of the atria or ventricles. Itis recognized that alternate lead systems may be substituted for thethree lead system illustrated in FIG. 1. While a particularmulti-chamber ICD and lead system is illustrated in FIG. 1,methodologies included in the present invention may adapted for use withany single chamber, dual chamber, or multi-chamber ICD or pacemakersystem, subcutaneous implantable device, or other internal or externalcardiac monitoring device.

FIG. 2 is a functional schematic diagram of the medical device ofFIG. 1. This diagram should be taken as exemplary of the type of devicewith which the invention may be embodied and not as limiting. Thedisclosed embodiment shown in FIG. 2 is a microprocessor-controlleddevice, but the methods of the present invention may also be practicedwith other types of devices such as those employing dedicated digitalcircuitry.

With regard to the electrode system illustrated in FIG. 1, ICD 10 isprovided with a number of connection terminals for achieving electricalconnection to the leads 6, 15, and 16 and their respective electrodes. Aconnection terminal 311 provides electrical connection to the housing 11for use as the indifferent electrode during unipolar stimulation orsensing. The connection terminals 320, 313, and 318 provide electricalconnection to coil electrodes 20, 8 and 23 respectively. Each of theseconnection terminals 311, 320, 313, and 318 are coupled to the highvoltage output circuit 234 to facilitate the delivery of high energyshocking pulses to the heart using one or more of the coil electrodes 8,20, and 23 and optionally the housing 11.

The connection terminals 317 and 321 provide electrical connection tothe helix electrode 17 and the ring electrode 21 positioned in the rightatrium. The connection terminals 317 and 321 are further coupled to anatrial sense amplifier 204 for sensing atrial signals such as P-waves.The connection terminals 326 and 324 provide electrical connection tothe helix electrode 26 and the ring electrode 24 positioned in the rightventricle. The connection terminals 326 and 324 are further coupled to aventricular sense amplifier 200 for sensing ventricular signals.

The atrial sense amplifier 204 and the ventricular sense amplifier 200preferably take the form of automatic gain controlled amplifiers withadjustable sensitivity. In accordance with the invention, ICD 10 and,more specifically, microprocessor 224 automatically adjusts thesensitivity of atrial sense amplifier 204, ventricular sense amplifier200 or both in response to detection of oversensing in order to reducethe likelihood of oversensing. Ventricular sense amplifier 200 andatrial sense amplifier 204 operate in accordance with originallyprogrammed sensing parameters for a plurality of cardiac cycles, andupon detecting oversensing, automatically provides the corrective actionto avoid future oversensing. In this manner, the adjustments provided byICD 10 to amplifiers 200 and 204 to avoid future oversensing are dynamicin nature. Particularly, microprocessor 224 increases a sensitivityvalue of the amplifiers, thus reducing the sensitivity, when oversensingis detected. Atrial sense amplifier 204 and ventricular sense amplifier200 receive timing information from pacer timing and control circuitry212.

Specifically, atrial sense amplifier 204 and ventricular sense amplifier200 receive blanking period input, e.g., ABLANK and VBLANK,respectively, which indicates the amount of time the electrodes are“turned off” in order to prevent saturation due to an applied pacingpulse or defibrillation shock. As will be described, the blankingperiods of atrial sense amplifier 204 and ventricular sense amplifier200 and, in turn, the blanking periods of sensing electrodes associatedwith the respective amplifiers may be automatically adjusted by ICD 10to reduce the likelihood of oversensing. The general operation of theventricular sense amplifier 200 and the atrial sense amplifier 204 maycorrespond to that disclosed in U.S. Pat. No. 5,117,824, by Keimel, etal., incorporated herein by reference in its entirety. Whenever a signalreceived by atrial sense amplifier 204 exceeds an atrial sensitivity, asignal is generated on the P-out signal line 206. Whenever a signalreceived by the ventricular sense amplifier 200 exceeds a ventricularsensitivity, a signal is generated on the R-out signal line 202.

Switch matrix 208 is used to select which of the available electrodesare coupled to a wide band amplifier 210 for use in digital signalanalysis. Selection of the electrodes is controlled by themicroprocessor 224 via data/address bus 218. The selected electrodeconfiguration may be varied as desired for the various sensing, pacing,cardioversion and defibrillation functions of the ICD 10. Specifically,microprocessor 224 may modify the electrode configurations based ondetection of oversensing due to cardiac or non-cardiac origins. Upondetection of R-wave oversensing, for example, microprocessor 224 maymodify the electrode configuration of the right ventricle from truebipolar sensing, e.g., tip-to-ring, to integrated bipolar sensing, e.g.,tip-to-coil.

Signals from the electrodes selected for coupling to bandpass amplifier210 are provided to multiplexer 220, and thereafter converted tomulti-bit digital signals by A/D converter 222, for storage in randomaccess memory 226 under control of direct memory access circuit 228 viadata/address bus 218. Microprocessor 224 may employ digital signalanalysis techniques to characterize the digitized signals stored inrandom access memory 226 to recognize and classify the patient's heartrhythm employing any of the numerous signal processing methodologiesknown in the art. An exemplary tachyarrhythmia recognition system isdescribed in U.S. Pat. No. 5,545,186 issued to Olson et al, incorporatedherein by reference in its entirety.

Upon detection of an arrhythmia, an episode of EGM data, along withsensed intervals and corresponding annotations of sensed events, arepreferably stored in random access memory 226. The EGM signals storedmay be sensed from programmed near-field and/or far-field sensingelectrode pairs. Typically, a near-field sensing electrode pair includesa tip electrode and a ring electrode located in the atrium or theventricle, such as electrodes 17 and 21 or electrodes 26 and 24. Afar-field sensing electrode pair includes electrodes spaced furtherapart such as any of: the defibrillation coil electrodes 8, 20 or 23with housing 11; a tip electrode 17 or 26 with housing 11; a tipelectrode 17 or 26 with a defibrillation coil electrode 20 or 23; oratrial tip electrode 17 with ventricular ring electrode 24. The use ofnear-field and far-field EGM sensing of arrhythmia episodes is describedin U.S. Pat. No. 5,193,535, issued to Bardy, incorporated herein byreference in its entirety. Annotation of sensed events, which may bedisplayed and stored with EGM data, is described in U.S. Pat. No.4,374,382 issued to Markowitz, incorporated herein by reference in itsentirety.

The telemetry circuit 330 receives downlink telemetry from and sendsuplink telemetry to an external programmer, as is conventional inimplantable anti-arrhythmia devices, by means of an antenna 332. Data tobe uplinked to the programmer and control signals for the telemetrycircuit are provided by microprocessor 224 via address/data bus 218. EGMdata that has been stored upon arrhythmia detection or as triggered byother monitoring algorithms may be uplinked to an external programmerusing telemetry circuit 330. Received telemetry is provided tomicroprocessor 224 via multiplexer 220. Numerous types of telemetrysystems known in the art for use in implantable devices may be used.

The remainder of the circuitry illustrated in FIG. 2 is an exemplaryembodiment of circuitry dedicated to providing cardiac pacing,cardioversion and defibrillation therapies. The pacer timing and controlcircuitry 212 includes programmable digital counters which control thebasic time intervals associated with various single, dual ormulti-chamber pacing modes or anti-tachycardia pacing therapiesdelivered in the atria or ventricles. Pacer circuitry 212 alsodetermines the amplitude of the cardiac pacing pulses under the controlof microprocessor 224.

During pacing, escape interval counters within pacer timing and controlcircuitry 212 are reset upon sensing of R-waves or P-waves as indicatedby signals on lines 202 and 206, respectively. In accordance with theselected mode of pacing, pacing pulses are generated by atrial paceroutput circuit 214 and ventricular pacer output circuit 216. The paceroutput circuits 214 and 216 are coupled to the desired electrodes forpacing via switch matrix 208. The escape interval counters are resetupon generation of pacing pulses, and thereby control the basic timingof cardiac pacing functions, including anti-tachycardia pacing.

The durations of the escape intervals are determined by microprocessor224 via data/address bus 218. The value of the count present in theescape interval counters when reset by sensed R-waves or P-waves can beused to measure R-R intervals and P-P intervals for detecting theoccurrence of a variety of arrhythmias.

The microprocessor 224 includes associated read-only memory (ROM) inwhich stored programs controlling the operation of the microprocessor224 reside. A portion of the random access memory (RAM) 226 may beconfigured as a number of recirculating buffers capable of holding aseries of measured intervals for analysis by the microprocessor 224 forpredicting or diagnosing an arrhythmia.

In response to the detection of tachycardia, anti-tachycardia pacingtherapy can be delivered by loading a regimen from microprocessor 224into the pacer timing and control circuitry 212 according to the type oftachycardia detected. In the event that higher voltage cardioversion ordefibrillation pulses are required, microprocessor 224 activates thecardioversion and defibrillation control circuitry 230 to initiatecharging of the high voltage capacitors 246 and 248 via charging circuit236 under the control of high voltage charging control line 240. Thevoltage on the high voltage capacitors is monitored via a voltagecapacitor (VCAP) line 244, which is passed through the multiplexer 220.When the voltage reaches a predetermined value set by microprocessor224, a logic signal is generated on the capacitor full (CF) line 254,terminating charging. The defibrillation or cardioversion pulse isdelivered to the heart under the control of the pacer timing and controlcircuitry 212 by an output circuit 234 via a control bus 238. The outputcircuit 234 determines the electrodes used for delivering thecardioversion or defibrillation pulse and the pulse wave shape.

In one embodiment, the ICD 10 may be equipped with a patientnotification system 150. Any patient notification method known in theart may be used such as generating perceivable twitch stimulation or anaudible sound. A patient notification system may include an audiotransducer that emits audible sounds including voiced statements ormusical tones stored in analog memory and correlated to a programming orinterrogation operating algorithm or to a warning trigger event asgenerally described in U.S. Pat. No. 6,067,473 issued to Greeninger etal., incorporated herein by reference in its entirety.

FIG. 3 is flowchart of a method for detecting atrial arrhythmias duringintermittent instances of ventricular pacing in a cardiac medical deviceaccording to an embodiment of the present disclosure. Flow chart 200 andother flow charts presented herein are intended to illustrate thefunctional operation of the device, and should not be construed asreflective of a specific form of software or hardware necessary topractice the invention. It is believed that the particular form ofsoftware will be determined primarily by the particular systemarchitecture employed in the device and by the particular detection andtherapy delivery methodologies employed by the device. Providingsoftware to accomplish the present invention in the context of anymodern IMD, given the disclosure herein, is within the abilities of oneof skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

As illustrated in FIG. 3, during detection of atrial arrhythmias, thedevice senses events, such as ventricular events, for example, Block300, and identifies the sensed ventricular event as being either anintrinsic sensed event Vs or a paced event Vp resulting from pacingbeing delivered by the device. Depending upon the number of RR intervalschosen for determining RR interval differences, the device determineswhether a predetermined number of sensed events, either a ventricularpacing event Vp or intrinsic ventricular sensed event VS, have beensensed, Block 302. For example, according to one embodiment, if thedesired number of RR intervals for RR interval differences is three, thepredetermined number of sensed events utilized in Block 302 would befour sensed events, with the four sensed events forming a sensingwindow, as will be illustrated below. If the predetermined number ofsensed events have not been sensed, the device determines the nextsensed event, Block 300, and the process is repeated.

Once the predetermined number of events are sensed, Yes in Block 302, asensed event window is identified based on the four events, Block 304,and a determination is made as to whether the number of the sensedevents in the sensed event window that are ventricular pace Vp events isless than or equal to a predetermined pacing event threshold, Block 306.For example, according to one embodiment, the pacing event threshold isset as one so that the device determines whether one or less of thesensed events in the sensed event window are ventricular pace events. Ifthe number of the sensed events in the sensed event window that areventricular pace Vp events is not less than or equal to, i.e., isgreater than the predetermined pacing event threshold, No in Block 306,the device determines the next sensed event, Block 300, and the processis repeated.

If the number of the sensed events in the sensed event window that areventricular pace Vp events is less than or equal to the predeterminedpacing event threshold, Yes in Block 306, the device determines whethereach of the RR intervals associated with the sensed events in thecurrent sensed event window are greater than a predetermined intervalthreshold, Block 308. For example, according to one embodiment thedevice determines whether each of the RR intervals associated with thesensed events in the sensed event window are greater than 220milliseconds. If each of the RR intervals associated with the sensedevents in the sensed event window are not greater than 220 milliseconds,No in Block 308, the device determines the next sensed event, Block 300,and the process is repeated using the next sensed event and theresulting next sensed event window.

If each of the RR intervals associated with the sensed events in thesensed event window are greater than 220 milliseconds, Yes in Block 308,the device determines differences or variability of the RR intervalsassociated with the sensed events in the sensed event window, Block 310,as will be described below. Once the RR intervals differences for thecurrent sensed event window have been determined in Block 308, thedevice determines whether a predetermined cardiac event timer hasexpired, Block 312. If the event timer has not expired, No in Block 312,the device determines the next sensed event, Block 300, and the processis repeated using the next sensed event and the resulting next sensedevent window. According to one embodiment, the cardiac event timer isset as two minutes so that once the event timer has expired, Yes inBlock 312, the device determines an atrial fibrillation AF score, Block314, based on the determined RR interval differences, Block 310,resulting from multiple sensed event windows occurring during thepredetermined time period, Block 312, i.e., two minutes for example. Thedetermination of the AF score is described below, with the device makinga determining of either an atrial fibrillation AF event or a non-atrialfibrillation event occurring based on a comparison of the AF score to anAF detection threshold. The stored differences are then cleared, Block316, and the device determines the next sensed event, Block 300, and theprocess is repeated for the next time period using the next sensedevents and the resulting next sensed event windows.

FIG. 4 is a schematic diagram illustrating detecting atrial arrhythmiasduring ventricular pacing in a cardiac medical device according to anembodiment of the present disclosure. As illustrated in FIGS. 3 and 4,according to one embodiment, once the device senses the predeterminednumber of sensed events 320-326, Yes in Block 302, a sensed event window332 is formed, Block 304, based on the current four sensed events320-326. The device determines whether only one or less of the sensedevents 320-326 are ventricular paced events, Block 306, and whether theRR intervals 338 formed between the sensed events 320-326 are greaterthan the interval threshold, Block 308. In the example illustrated inFIG. 4, all of sensed events 320-326 are ventricular sensed Vs events,and assuming all of the intervals 338 formed by the sensed events320-326 are greater than the interval threshold, Yes in Block 308, thedevice determines and stores an interval difference factor associatedwith the intervals 338 of the current sensed events 320-326, Block 310.If all of the intervals 338 formed by the sensed events 320-326 are notgreater than the interval threshold, No in Block 308, the devicedetermines the next sensed event, Block 300, and the process is repeatedusing the next sensed event and the resulting next sensed event window.

Assuming the cardiac event timer has not yet expired, No in Block 312,the device senses the next event 328, Block 300, and a sensed eventwindow 334 is formed, Block 304, based on the current four sensed events322-328. The device determines whether only one or less of the sensedevents 322-328 are ventricular paced events, Block 306, and whether theRR intervals 338 formed between the sensed events 322-328 are greaterthan the interval threshold, Block 308. In the example illustrated inFIG. 4, since only one sensed event 338 of sensed events 322-328 is aventricular paced Vp event, and assuming all of the intervals 338 formedby the sensed events 322-328 are greater than the interval threshold,Yes in Block 308, the device determines and stores an intervaldifference factor associated with the intervals 338 of the currentsensed events 322-328, Block 310.

Assuming the cardiac event timer has not yet expired, No in Block 312,the device senses the next event 330, Block 300, and a sensed eventwindow 336 is formed, Block 304, based on the current four sensed events324-330. The device determines whether only one or less of the sensedevents 324-330 are ventricular paced events, Block 306, and whether theRR intervals 338 formed between the sensed events 324-330 are greaterthan the interval threshold, Block 308. In the example illustrated inFIG. 4, since two sensed events 338 and 340 of sensed events 324-330 areventricular paced Vp events, and therefore the number of sensed eventsin the sensed event window 336 that are ventricular paced Vp events isnot less than or equal to the pacing event threshold, No in Block 306,an RR interval difference factor is not determined for that sensed eventwindow 336, and the device determines the next sensed event, Block 300,and the process is repeated using the next sensed event and theresulting next sensed event window, and so on until the timer hasexpired, Yes in Block 312. Once the timer has expired, Yes in Block 312,the atrial fibrillation AF score for that time period is determinedbased on the currently stored interval difference factors, as describedbelow.

FIG. 5 is a schematic diagram illustrating detecting atrial arrhythmiasduring ventricular pacing in a cardiac medical device according toanother embodiment of the present disclosure. As illustrated in FIGS. 3and 5, according to another embodiment, once the device senses thepredetermined number of sensed events 340-346, Yes in Block 302, asensed event window 356 is formed, Block 304, based on the current foursensed events 340-346. The device determines whether only one or less ofthe sensed events 340-346 are ventricular paced Vp events, Block 306,and whether the RR intervals 366 formed between the sensed events340-346 are greater than the interval threshold, Block 308. In theexemplary embodiment illustrated in FIG. 5, during the determination asto whether only one or less of the sensed events 340-346 are ventricularpaced Vp events, Block 306, rather than making the determination basedon all of the sensed events 340-346 in the sensed event window 356, thedevice determines whether one or more of a predetermined number of thesensed events 340-346 are ventricular pace Vp events. For example,according to one embodiment, the device may determine whether only oneor less of the most recent sensed event 346 in the sensed event window356 and the previous two sensed events 342 and 344 are ventricularsensed Vp events, Block 306.

In the example illustrated in FIG. 5, the most recent sensed event 346in the sensed event window 356 is a ventricular pace Vp event, and thetwo previous sensed events 342 and 344 are both ventricular sense VSevents, resulting in there being only one ventricular pace Vp event.Therefore, the number of ventricular pace VP events is determined to beless than or equal to the ventricular pace Vp event threshold, i.e., oneventricular pace Vp event, Yes in Block 306. As a result, similar toabove, the device determines whether the RR intervals 366 associatedwith the current sensed events 340-346 are greater than an intervalthreshold, Block 308, such as 220 milliseconds, for example. If the RRintervals 366 are not greater than the interval threshold, No in Block308, the device does not store an interval difference factor, Block 310,for the intervals 366 associated with the current sensed events 340-346,and the process is repeated using the next sensed event 348 and theresulting next sensed event window 358.

If each of the RR intervals 366 are greater than the interval threshold,Yes in Block 308, the device stores an interval difference factor, Block310, associated with the intervals 366 formed between the current sensedevents 340-346, described below, and, assuming the timer has notexpired, No in Block 312, the process is repeated using the next sensedevent 348 and the resulting next sensed event window 360. If the timerhas expired, Yes in Block 312, the device determines an atrialfibrillation AF score, Block 314, based on the determined RR intervaldifference factors, Block 310, resulting from multiple sensed eventwindows over the predetermined time period of Block 312, such as twominutes, for example. The determination of the AF score is describedbelow, with the device making a determining of either an atrialfibrillation AF event or a non-atrial fibrillation event occurring basedon a comparison of the AF score to an AF detection threshold. Thecurrent counters are then cleared, Block 316, and the device determinesthe next sensed event, Block 300, and the process is repeated for thenext time period using the next sensed events and the resulting nextsensed event windows.

As described above, if the RR intervals are not greater than theinterval threshold, No in Block 310, or if the cardiac event timer hasnot yet expired, No in Block 312, the device senses the next cardiacevent 348, Block 300, and a sensed event window 358 is formed, Block304, based on the most current four sensed events 342-348. The devicedetermines whether only one or less of the most recent sensed event 348in the sensed event window 358 and the previous two sensed events 344and 346 are ventricular sensed Vp events, Block 306. In the exampleillustrated in FIG. 5, the most recent sensed event 348 and one sensedevent 346 of the two previous sensed events 344 and 346 are ventricularpace Vp events, and the other previous sensed event 344 is a ventricularsense VS event, resulting in there being two ventricular pace Vp events.Therefore, since the number of ventricular pace VP events is not lessthan or equal to the ventricular pace Vp event threshold, No in Block306, the device does not determine and store an interval differencefactor, Block 310, for the intervals 366 formed by the current sensedevents 342-348, and the process is repeated using the next sensed event350 and the resulting next sensed event window 360.

In particular, the device determines whether only one or less of themost recent sensed event 350 in the sensed event window 360 and theprevious two sensed events 346 and 348 are ventricular sensed Vp events,Block 306. In the example illustrated in FIG. 5, the most recent sensedevent 350 is a ventricular sense Vs event and both of the previous twosensed events 346 and 348 are ventricular pace Vp events, resulting inthere being two ventricular pace Vp events occurring during the sensedevent window 360. As a result, the number of ventricular pace Vp eventsis not less than or equal to the ventricular pace Vp event threshold, Noin Block 306, and therefore the device does not store an intervaldifference factor associated with the intervals 366 formed by thecurrent sensed events 344-350, and the process is repeated using thenext sensed event 352 and the resulting next sensed event window 362.

In particular, the device determines whether only one or less of themost recent sensed event 352 in the sensed event window 362 and theprevious two sensed events 348 and 350 are ventricular sensed Vp events,Block 306. In the example illustrated in FIG. 5, the most recent sensedevent 352 and one sensed event 350 of the two previous sensed events 348and 350 are ventricular sense Vs events, and the other previous sensedevent 348 is a ventricular pace Vp event, resulting in only oneventricular pace Vp event occurring during the sensed event window 362.As a result, the number of ventricular pace VP events is less than orequal to the ventricular pace Vp event threshold, Yes in Block 306, andtherefore the device determines whether the RR intervals 366 associatedwith the current sensed events 346-352 are greater than the intervalthreshold, Block 308. If the RR intervals 366 are not greater than theinterval threshold, No in Block 308, the device does not store aninterval difference factor, Block 310, associated with the intervals 366formed by the current sensed events 346-352, and the process is repeatedusing the next sensed event 354 and the resulting next sensed eventwindow 364.

If each of the RR intervals 366 are greater than the interval threshold,Yes in Block 308, the device stores an interval difference factor, Block310, associated with the intervals 366 formed between the current sensedevents 346-352, described below. Assuming the timer has not expired, Noin Block 312, the process is then repeated using the next sensed event354 and the resulting next sensed event window 364. If the timer hasexpired, Yes in Block 312, the device determines an atrial fibrillationAF score, Block 314, based on the determined RR interval differencefactors, Block 310, resulting from multiple sensed event windows overthe predetermined time period of Block 312, i.e., two minutes forexample. The determination of the AF score is described below, with thedevice making a determining of either an atrial fibrillation AF event ora non-atrial fibrillation event occurring based on a comparison of theAF score to an AF detection threshold. The counters are then cleared,Block 316, and the device determines the next sensed event, Block 300,and the process is repeated for the next time period using the nextsensed events and the resulting next sensed event windows, and so on.

In the example illustrated in FIG. 5, the most recent sensed event 354and both of the two previous sensed events 350 and 352 are ventricularsense Vs events, resulting in the number of ventricular pace VP eventsbeing less than or equal to the ventricular pace Vp event threshold, Yesin Block 306. Assuming that each of the RR intervals 366 are greaterthan the interval threshold, Yes in Block 308, the device determines andstores an interval difference factor, Block 310, associated with theintervals 366 formed between the current sensed events 348-354,described below. In this way, assuming the timer has expired, Yes inBlock 312, in the example of FIG. 5, the device determines and stores aninterval difference factor, Block 310, only for intervals formed insensed event windows 356, 362 and 364, and not for intervals formed insensed event windows 358 and 360. The determination of the atrialfibrillation AF score, Block 314, described below, is therefore madebased on the interval difference factor, Block 310, determined only forintervals formed in sensed event windows 356, 362 and 364, and thereforedoes not include intervals formed in sense event windows 358 and 360having more than the predetermined number of ventricular pace Vp eventstherein.

FIG. 6 is a schematic diagram of classifying of cardiac events in acardiac medical device according to an embodiment of the presentdisclosure. As illustrated in FIG. 6, according to one embodiment, inorder to determine the atrial fibrillation AF score based on thedetermined RR intervals difference factors resulting from multiplesensed event windows, described above, the determined RR intervaldifference factors calculated for the RR intervals formed by the sensedevents for each sensed event window, described above, are used to plotsingle points on a Lorentz plot 14.

The Lorenz plot 14 is a Cartesian coordinate system defined by δRR_(i)along the x-axis 18 and δRR_(i-1) along the y-axis 16. As such, eachplotted point in a Lorenz plot is defined by an x-coordinate equalingδRR_(i) and a y-coordinate equaling δRR_(i-1). δRR_(i) is the differencebetween the i^(th) RR interval and the previous RR interval, RRI_(i-1).δRR_(i-1) is the difference between RRI_(i-1) and the previous RRinterval, RRI_(i-2). As such, each data point plotted on the Lorenz plot14 represents a ventricular cycle length VCL pattern relating to threeconsecutive VCLs: RRI_(i), RRI_(i-1) and RRI_(i-2), measured between thefour consecutively sensed R-waves associated with a sensing eventwindow.

In order to plot each point on the Lorenz plot area 14, a (δRR_(i),δRR_(i-1)) point is identified based on the RR interval differencedetermined for the intervals formed by the sensed events in each singlesensed event window during the two minute time period having one or lessventricular pace Vp events, described above. The atrial fibrillation AFscore for each two minute time period is then determined based on therelative position of the resulting plotted points on the plot area 14.For example, using the example illustrated in FIG. 5, a first data point23 is plotted based on the RR interval difference factor determined forintervals 366 formed in sensed event window 356, a second data point 25is plotted based on the RR interval difference factor determined forintervals 366 formed in sensed event window 362, and a third data point27 is plotted based on the RR interval difference factor determined forintervals 366 formed in sensed event window 364, and so forth.

In particular, for example, δRR_(i) for the first data point 23 isdetermined as the difference between the RR interval 366 between sense346 and sense 344 and the RR interval 366 between sense 344 and sense342, and δRR_(i-1) is determined as the difference between the RRinterval 366 between sense 344 and sense 342 and the RR interval 366between sense 342 and sense 340. In the same way, the corresponding(δRR_(i), δRR_(i-1)) point is identified for sensed event windows 362and 364, and so on until the timer has expired.

The plotted (δRR_(i), δRR_(i-1)) points over a two minute time periodare then used to identify the event as either an atrial fibrillationevent or a non-atrial fibrillation. Methods have been developed fordetecting atrial arrhythmias based on the irregularity of ventricularcycles measured by RR intervals that exhibit discriminatory signatureswhen plotted in a Lorenz scatter plot such as the plot shown in FIG. 6.One such method is generally disclosed by Ritscher et al. in U.S. Pat.No. 7,031,765, or in U.S. Pat. No. 8,639,316 to Sarkar, bothincorporated herein by reference in their entireties. Other methods aregenerally disclosed by Sarkar, et al. in U.S. Pat. No. 7,623,911 and inU.S. Pat. No. 7,537,569 and by Houben in U.S. Pat. No. 7,627,368, all ofwhich patents are also incorporated herein by reference in theirentirety.

FIG. 7 is a diagram of an exemplary two-dimensional histogramrepresenting a Lorenz plot area for identifying cardiac events.Generally, the Lorenz plot area 14 shown in FIG. 7 is numericallyrepresented by a two-dimensional histogram 160 having predefined ranges166 and 164 in both positive and negative directions for the δRR_(i) andδRR_(i-1) coordinates, respectively. The two-dimensional histogram isdivided into bins 168 each having a predefined range of δRR_(i) andδRR_(i-1) values. In one example, the histogram range might extend from−1200 ms to +1200 ms for both δRR_(i) and δRR_(i-1) values, and thehistogram range is divided into bins extending 7.5 ms in each of the twodimensions resulting in a 160 bin×160 bin histogram. The successive RRIdifferences determined over a detection time interval are used topopulate the histogram 160. Each bin stores a count of the number of(δRR_(i), δRR_(i-1)) data points falling into the bin range. The bincounts may then be used in determining RRI variability metrics andpatterns for determining a cardiac rhythm type.

An RRI variability metric is determined from the scatter plot.Generally, the more histogram bins that are occupied, i.e. the moresparse the distribution of (δRR_(i), δRR_(i-1)) points, the moreirregular the VCL during the data acquisition time period. As such, ametric of the RRI variability can be used for detecting atrialfibrillation, which is associated with highly irregular VCL. In oneembodiment, an RRI variability metric for detecting AF, referred to asan AF score is computed as generally described in the above-incorporated'911 patent. Briefly, the AF score may be defined by the equation:AF Evidence=Irregularity Evidence−Origin Count−PAC Evidence

wherein Irregularity Evidence is the number of occupied histogram binsoutside a Zero Segment defined around the origin of the Lorenz plotarea. During normal sinus rhythm or highly organized atrial tachycardia,nearly all points will fall into the Zero Segment because of relativelysmall, consistent differences between consecutive RRIs. A high number ofoccupied histogram bins outside the Zero segment is therefore positiveevidence for AF.

The Origin Count is the number of points in a “Zero Segment” definedaround the Lorenz plot origin. A high Origin Count indicates regularRRIs, a negative indicator of atrial fibrillation, and is thereforesubtracted from the Irregularity Evidence term. In addition, a regularPAC evidence score may be computed as generally described in theabove-incorporated '911 patent. The regular PAC evidence score iscomputed based on a cluster signature pattern of data points that isparticularly associated with PACs that occur at regular couplingintervals and present regular patterns of RRIs, e.g. associated withbigeminy (short-short-long RRIs) or trigeminy (short-short-short-longRRIs).

In other embodiments, an AF score or other RRI variability score forclassifying an atrial rhythm may be computed as described in any of theabove-incorporated '765, '316, '911, '569 and '368 patents.

The AF score is compared to an AF threshold for detecting atrialfibrillation to determine whether the AF score corresponds to an AFevent. The AF threshold may be selected and optimized based onhistorical clinical data of selected patient populations or historicalindividual patient data, and the optimal threshold setting may vary frompatient to patient. If the metric crosses a detection threshold, AFdetection occurs. A response to AF detection is made, either in responseto a classification of a single two second time interval as being AF,i.e., being greater than the AF threshold, or in response to apredetermined number of two second intervals being classified as beingan AF event by each being greater than the AF threshold. Such responseto the AF detection may include withholding or altering therapy, such asa ventricular therapy, for example, storing data that can be laterretrieved by a clinician, triggering an alarm to the patient or that maybe sent remotely to alert the clinician, delivering or adjusting atherapy, and triggering other signal acquisition or analysis.

The RRI measurements may continue to be performed after an AF detectionto fill the histogram during the next detection time interval. Aftereach detection time interval, the RRI variability metric is determinedand the histogram bins are re-initialized to zero for the next detectiontime interval. The new RRI variability metric determined at the end ofeach data acquisition interval may be used to determine if the AFepisode is sustained or terminated.

FIG. 8 is a flowchart of classification of an arrhythmia according to anembodiment of the disclosure. According to another embodiment, once thetwo minute time period has expired and the plot has been populated witha point associated with each determined RR interval difference factordetermined based on the intervals in each sensed event window occurringduring the two minute time period, the device determines whether toclassify the event during that two minute time period as being either anatrial fibrillation AF event, a non-atrial fibrillation event, or anunclassified event. For example, the device may look at any one or moreof several factors, in any combination or particular order, to determinethat the event should be determined to be an unclassified event, i.e.,that the event can neither be classified as an AF event or a non-AFevent. Therefore, as illustrated in FIGS. 3 and 8, the device maydetermine the number of valid RR interval pairs, i.e., three validconsecutive RR intervals, formed between the predetermined number ofsensed events that were generated during the two minute time period,Block 400. In particular, the device determines the number of sensedevent windows containing three consecutive RR intervals that were formedin Block 304 during the two minute time period, Block 312.

A determination is then made as to whether the total number of validthree consecutive RR intervals formed during the two minute time periodis greater than an interval pair threshold, Block 402. According to anembodiment, the predetermined number of valid interval pairs is set as30 valid interval pairs. If the number of sensed event windows that areformed during the two minute time period is less than the interval pairthreshold, Yes in Block 402, meaning less than 30 sensed event windowswere determined during the two minute time period, and therefore lessthan 30 interval difference factors were determined in Block 310 duringthe two minute time interval, resulting in the plot being populated withless than 30 points (assuming the RR interval threshold is alsosatisfied in Block 308), the two minute time period is determined to beunclassified, Block 404.

If the number of sensed event windows that are formed during the twominute time period is not less than 30, No in Block 402, meaning 30 ormore sensed event windows were determined during the two minute timeperiod, and therefore 30 or more interval difference factors weredetermined in Block 310 during the two minute time interval, resultingin the plot being populated with 30 or more points, the interval pairsfactor is determined not to be satisfied as an indication of the twominute time interval being unclassified.

According to another embodiment, the device may determine the totalnumber of RR intervals that were determined to be less than the intervalthreshold in Block 308 during the two minute time period, Block 410,either alone or in combination with one or more other factors. Thedevice determines whether a predetermined number of RR intervals weredetermined to be greater than the RR interval threshold during the twominute time period, Block 412, such as 10 RR intervals, for example.

If more than 10 RR intervals of the total number of RR intervals thatwere determined during the two minute time period were greater than theRR interval threshold, Yes in Block 412, the interval length factor isdetermined to be satisfied and the two minute time period is determinedto be unclassified, Block 404. If 10 RR intervals or less were greaterthan the RR interval threshold during the two minute time period, No inBlock 412, the interval length factor is determined not to be satisfiedas an indication of the two minute time interval being unclassified.

In order to classify the two minute time period, the device may alsodetermine a short interval count of the total number of RR intervalsfrom all of the sensing windows obtained during the two minute timeperiod that were less than or equal to a predetermined short intervalrate, Block 414, such as 120 milliseconds or 130 milliseconds, forexample. The device determines whether the short interval count isgreater than a short interval rate threshold, Block 416, such as 5 shortintervals for example.

If the determined short interval count is greater than the shortinterval rate threshold, Yes in Block 416, the short interval countfactor is satisfied as an indication of the two minute time intervalbeing unclassified and therefore the two minute time interval isdetermined to be unclassified, Block 404. On the other hand, if thedetermined short interval count is not greater than the short intervalrate threshold, No in Block 416, the short interval count is determinednot to be satisfied as an indication of the two minute time intervalbeing unclassified.

The device may also determine the number of sensed events, sensed duringthe total two minute time period within all of the sensed event windowsthat were determined to be ventricular pace Vp events, Block 418. Adetermination is made as to whether the determined number of ventricularpace Vp sensed during the two minute time interval is greater than atotal ventricular pace Vp event threshold, Block 420. According to oneembodiment, the total ventricular pace Vp threshold is set as 30ventricular pace Vp events, for example.

If the number of ventricular pace Vp sensed during the two minute timeinterval is greater than the total ventricular pace Vp event threshold,Yes in Block 420, the ventricular pace factor is satisfied as anindication of the two minute time interval being unclassified andtherefore the two minute time interval is determined to be unclassified,Block 404. On the other hand, if the determined short interval count isnot greater than the short interval rate threshold, No in Block 416, theshort interval count is determined not to be satisfied as an indicationof the two minute time interval being unclassified.

The device may also determine whether a determination of oversensingcaused by noise was met, or in process during the two minute timeperiod, Block 422. The determination of oversensing may be performed bythe device using any known oversensing determination scheme, such as theoversensing determination describe in U.S. Pat. No. 7,333,855 toGunderson et. al., incorporated herein by reference in it's entirety. Ifa determination of oversensing was met or was in process during the twominute time period, Yes in Block 422, the oversensing factor issatisfied as an indication of the two minute time interval beingunclassified and therefore the two minute time interval is determined tobe unclassified, Block 404. If a determination of oversensing was notmet or was not in process during the two minute time period, No in Block422, the oversensing factor is not satisfied as an indication of the twominute time interval being unclassified.

Finally, the device may determine whether a determination of T-waveoversensing was met or in process during the two minute time period,Block 424. The determination of T-wave oversensing may be performed bythe device using any known T-wave oversensing determination scheme, suchas the T-wave oversensing determination describe in U.S. Pat. No.7,831,304 to Gillberg, et al., incorporated herein by reference in it'sentirety. If a determination of T-wave oversensing was met or was inprocess during the two minute time period, Yes in Block 424, the T-waveoversensing factor is satisfied as an indication of the two minute timeinterval being unclassified and therefore the two minute time intervalis determined to be unclassified, Block 404. If a determination ofT-wave oversensing was not met or was not in process during the twominute time period, No in Block 424, the T-wave oversensing factor isnot satisfied as an indication of the two minute time interval beingunclassified.

In this way, the device may use one or more of the described factors,which if satisfied would cause the device to determine the two minutetime period as being unclassified, and if at least one the describedfactors for identifying the two minute time period as being unclassifiedare met, the two minute time period is identified as being unclassified,Block 426. If none of the factors are satisfied, the AF score isdetermined based on the populated plot, and a determination is made asto whether the AF score is greater than an AF threshold, Block 426, asdescribed above. If the AF score is greater than the AF threshold, Yesin Block 426, the event for the two minute period is classified as an AFevent, Block 406. On the other hand, if the AF score is not greater thanthe AF threshold, No in Block 426, the event for the two minute periodis classified as a non AF event, Block 408.

It is understood that the determination of whether the event is anunclassified event, Block 404, or an atrial fibrillation event, Block406 or a non-atrial fibrillation event, Block 408, may be made in anyorder, or at the same time, so that the determination of the two minutetime period as being an unclassified event may be used to override aninitial determination of the two minute time period as being either anatrial fibrillation event or a non-atrial fibrillation event, or madeprior to determining the two minute time period as an atrialfibrillation event or a non-atrial fibrillation event.

FIG. 9 is a schematic diagram of determination of a cardiac event,according to the present disclosure. As illustrated in FIG. 9, thedevice identifies each two minute interval as being either an AF event,a non AF event or an unclassified event using the method above in FIG.8, and utilizes the identifications resulting for the two minute timeperiods to detect an AF episode. For example, once a predeterminednumber of two second windows, such as three, for example, have beenidentified as an AF event, the device determines that an atrialfibrillation episode is occurring. Therefore, as illustrated in thescenario of timing diagram (a) of FIG. 9, once the three two minute timeintervals, 500-504, are classified as an AF event, the device determinesthat atrial fibrillation is detected. However, in the scenarioillustrated in timing diagram (b), two consecutive two minute intervals506 and 508 are identified as being AF events, a subsequent two minutetime interval 510 is identified as being unclassified, which is followedby a subsequent interval 512 being identified as an AF event. Accordingto an embodiment, the device may ignore the unclassified two minute timeinterval 510 and determine an AF episode once the third time interval512 identified as AF occurs, so that an AF episode may be identifieddespite intermittent unclassified two minute time intervals occurring.

As illustrated in the timing diagram of scenario (c), during thedetermination of whether the predetermined number of two second windowsare identified as AF events, the device updates an AF event counter eachtime an AF event is determined. For example, at the identification oftwo minute interval 514, the AF event counter is increment to one, andat the identification of subsequent two minute interval 514, the AFevent counter is incremented to two. If two minute interval 518 werealso identified as an AF event, the device would determine an AFepisode, since three two minute intervals identified as an AF eventwould have occurred. However, since two minute interval 518 wasidentified as a non-AF event, the episode is determined to haveterminated, and the AF counter is reset to zero, and the processcontinues with the next two second time interval 520. In the timingdiagram of scenario (c), at the identification of two minute interval506, the AF event counter is increment to one, at the identification ofsubsequent two minute interval 508, the AF event counter is incrementedto two, at the identification of subsequent two minute interval 510,since the event was determined to be unclassified, the event counterremains as being equal to two, and at the identification of subsequenttwo minute interval 512, the AF event counter is incremented to three,and an AF episode is determined.

Similarly, in the timing diagram of scenario (d), at the identificationof two minute interval 522, the AF event counter is increment to one, atthe identification of subsequent two minute interval 524, the AF eventcounter is incremented to two, at the identification of subsequent twominute intervals 526 and 528, since the event was determined to beunclassified, the unclassified event count is incremented and the AFevent count remains as being equal to two, and at the identification ofsubsequent two minute interval 530, the AF event count is incremented tothree, and an AF episode is determined.

Had any of intervals 524-530 been identified as non-AF, indicating thetermination of the AF episode, the AF event counter would have been setto zero and the process repeated starting with the next classified twominute interval. However, in addition to a two minute interval beingidentified as a non-AF event, the episode may also be determined to haveterminate and the AF count is reset to zero if a predetermined number oftwo minute time periods are identified as unclassified, such as five twominute time periods, for example. Therefore, as illustrated in thetiming diagram of scenario (e), at the identification of two minuteinterval 532, the AF event count is increment to one, at theidentification of subsequent two minute interval 534, the AF event countis incremented to two, at the identification of the four subsequent twominute intervals 536-542, since the event was determined to beunclassified, the AF event count remains equal to two. If the subsequenttwo minute interval 544 is determined to either unclassified or a non-AFevent, the AF episode would be determined to have terminated and the AFevent counter would be set to zero and the process repeated startingwith the next classified two minute interval. If two minute time period546 had been classified as an AF event, an AF episode would have beenidentified.

FIG. 10 is a flowchart of determination of a cardiac episode, accordingto an embodiment of the disclosure. As illustrated in FIGS. 9 and 10,during identification of two minute intervals as being either an AFevent, a non AF event or an unclassified event using the methoddescribed above in FIG. 8, once the identification of a two minute timeinterval as an AF event occurs, Block 600, and the AF event counter isincremented, Block 602, and the device determines whether the AF eventcount is equal to an AF event count threshold, Block 604, such as threeAF events, for example. Once the AF event count is equal to the AF eventcount threshold, Yes in Block 604, an AF episode is identified, Block606. If the AF event count is not equal to the AF event count threshold,No in Block 604, the device determines, based on the next two minutetime interval classification, Block 608, whether the next two minutetime interval is classified as a non-AF event, Block 612. If the nexttwo minute time interval is classified as a non-AF event, Yes in Block612, the AF event counter and the unclassified event counter are bothset to zero, Block 614, and the process repeated starting with the nextclassified two minute interval, Block 600.

If the next two minute time interval is not classified as a non-AFevent, No in Block 612, the device determines whether the unclassifiedevent counter is equal to the unclassified event count threshold, suchas five unclassified events, for example, Block 616. If the unclassifiedevent counter is not equal to the unclassified event count threshold, Noin Block 616, the process is repeated based on the next subsequent twominute time interval classification, Block 608. If the unclassifiedevent counter is equal to the unclassified event count threshold, Yes inBlock 616, the AF event counter and the unclassified event counter areboth set to zero Block 614, and the process repeated starting with thenext classified two minute interval, Block 600.

According an embodiment of the disclosure, once the AF event count isequal to the AF event threshold, Yes in Block 604, and therefore an AFepisode is identified in Block 606, the device may determine the end ofthe specific AF episode. For example, at identification of an AFepisode, Block 606, the unclassified event counter is set to zero, Block618, the device determines, based on the next two minute time periodclassification, Block 620, whether the next two minute time interval isclassified as a non-AF event, Block 622. If the next two minute timeinterval is classified as a non-AF event, Yes in Block 622, the episodeis determine to have terminated, Block 624, and the AF event counter andthe unclassified event counter are both set to zero Block 614, and theprocess is repeated starting with the next classified two minuteinterval, Block 600. If the next two minute time interval is notclassified as a non-AF event, No in Block 622, the device determineswhether the unclassified event counter is equal to the unclassifiedevent count threshold, Block 626. If the unclassified event counter isnot equal to the unclassified event count threshold, No in Block 626,the process is repeated based on the next subsequent two minute timeperiod classification, Block 620. If the unclassified event counter isequal to the unclassified event count threshold, Yes in Block 626, theAF episode is determined to have terminated, Block 624, the AF eventcounter and the unclassified event counter are both set to zero Block614, and the process repeated starting with the next classified twominute interval, Block 600.

According to an embodiment of the disclosure, the identification of theAF episode, along with the classification of the two minute timeintervals as being either an AF event, a non-AF event, or anunclassified event are stored and may be later retrieved by a clinician,either remotely or through interrogation of the device. AF burden (e.g.,an AF daily burden) may be calculated using a single two minute AFclassification or a predetermined number of two minute AFclassifications, such as three two minute AF classifications, forexample. An alarm may be sent remotely to alert the clinician if the AFburden exceeds a predetermined threshold (e.g., one hour for example),or may be sent to notify the clinician or patient of the one or more twominute time interval classifications and/or the identification of an AFepisode, when termination of the episode occurs and how it wasdetermined to have terminated, i.e., by a non-AF two minute time periodor the predetermined number of unclassified two minute time periods.

Thus, an apparatus and method have been presented in the foregoingdescription with reference to specific embodiments. It is appreciatedthat various modifications to the referenced embodiments may be madewithout departing from the scope of the invention as set forth in thefollowing claims.

The invention claimed is:
 1. A method comprising: obtaining a cardiacsignal sensed via one or more electrodes; identifying an atrialtachyarrhythmia episode is occurring based on analysis of the cardiacsignal; determining, for each time interval of a plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode, one or more classification factorsassociated with the cardiac signal sensed during each of the respectiveplurality of predetermined time intervals, wherein each time interval ofthe plurality of time intervals extends for an amount of time, andwherein the amount of time corresponding to a first time interval of theplurality of time intervals is the same as the amount of timecorresponding to each other time interval of the plurality of timeintervals; identifying, for each time interval of the plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode, the cardiac signal sensed during each ofthe respective plurality of predetermined time intervals as one of anatrial fibrillation (AF) event, a non-AF event, or an unclassified eventbased on the one or more determined classification factors, wherein anunclassified event represents an event which is not classified as an AFevent or a non-AF event; maintaining a count of a number of timeintervals of the plurality of predetermined time intervals occurringafter identification of the atrial tachyarrhythmia episode that areidentified as unclassified events; determining the count exceeds anunclassified event count threshold; detecting termination of the atrialtachyarrhythmia episode in response to determining the count exceeds theunclassified event count threshold; and storing, in a computer-readablemedium, information related to the atrial tachyarrhythmia episode. 2.The method of claim 1, further comprising providing the informationrelated to the atrial tachyarrhythmia episode to a user.
 3. The methodof claim 1, wherein the atrial tachyarrhythmia episode comprises an AFepisode, the method further comprising: calculating an AF burden basedon at least the information related to the AF episode; determining thatthe AF burden exceeds a predetermined threshold; and providing an alertto a user in response to determining that the AF burden exceeds thepredetermined threshold.
 4. The method of claim 1, further comprising:identifying one of the plurality of predetermined time intervalsoccurring after identification of the atrial tachyarrhythmia episode asa non-AF event; and detecting termination of the atrial tachyarrhythmiaepisode in response to identifying one of the plurality of predeterminedtime intervals occurring after identification of the atrialtachyarrhythmia episode as the non-AF event.
 5. The method of claim 1,wherein determining the one or more classification factors associatedwith the cardiac signal comprises determining one or more of aventricular pacing factor, a noise oversensing factor, and a T-waveoversensing factor; and wherein identifying the cardiac signal sensedduring a respective one of the plurality of predetermined time intervalsas the unclassified event based on one or more of: the ventricularpacing factor indicating ventricular pacing that exceeds a ventricularpacing threshold; the noise oversensing factor indicating oversensingcaused by noise; and the T-wave oversensing factor indicating T-waveoversensing.
 6. The method of claim 1, wherein identifying the signal asthe unclassified event based on the classification factors comprises:determining sensed event windows having predetermined interval pairs inresponse to the sensed cardiac signal during a predetermined timeperiod; determining whether a number of the determined sense eventwindows is less than an interval pair threshold; determining whether anumber of intervals occurring during the predetermined time period andnot within the sensed event windows that are less than a predeterminedinterval threshold is greater than a sensed interval threshold; anddetermining whether a predetermined number of intervals occurring duringthe predetermined time period and not within the sensed event windowsare less than a short interval count threshold.
 7. The method of claim1, wherein the amount of time corresponding to each time interval of theplurality of time intervals is within a range from one minute to fiveminutes.
 8. The method of claim 1, wherein the amount of timecorresponding to each time interval of the plurality of time intervalsis two minutes.
 9. A medical device comprising: a memory; a senseamplifier configured to obtain a cardiac signal sensed via one or moreelectrodes; and a processor configured to: identify an atrialtachyarrhythmia episode is occurring based on analysis of the cardiacsignal; determine, for each time interval of a plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode, one or more classification factorsassociated with the cardiac signal sensed during each of the respectiveplurality of predetermined time, wherein each time interval of theplurality of time intervals extends for an amount of time, and whereinthe amount of time corresponding to a first time interval of theplurality of time intervals is the same as the amount of timecorresponding to each other time interval of the plurality of timeintervals; identify, for each time interval of the plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode, the cardiac signal sensed during each ofthe respective plurality of predetermined time intervals as one of anatrial fibrillation (AF) event, a non-AF event, or an unclassified eventbased on the one or more determined classification factors, wherein anunclassified event represents an event which is not classified as an AFevent or a non-AF event; maintain a count of a number of time intervalsof the plurality of predetermined time intervals occurring afteridentification of the atrial tachyarrhythmia episode that are identifiedas unclassified events; determine the count exceeds an unclassifiedevent count threshold; detecting termination of the atrialtachyarrhythmia episode in response to determining the count exceeds theunclassified event count threshold; and store, in a computer-readablemedium, information related to the atrial tachyarrhythmia episode. 10.The device of claim 9, further comprising a telemetry circuit, whereinthe processor is further configured to control the telemetry circuit toprovide the information related to the atrial tachyarrhythmia episode toa user.
 11. The device of claim 9, wherein the atrial tachyarrhythmiaepisode comprises an AF episode, the processor further configured to:calculate an AF burden based on at least the information related to theAF episode; determine that the AF burden exceeds a predeterminedthreshold; and control the telemetry circuit to provide an alert to auser in response to determining that the AF burden exceeds thepredetermined threshold.
 12. The device of claim 9, wherein theprocessor is further configured to: identify one of the plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode as a non-AF event; and detect terminationof the atrial tachyarrhythmia episode in response to identifying one ofthe plurality of predetermined time intervals occurring afteridentification of the atrial tachyarrhythmia episode as the non-AFevent.
 13. The device of claim 9, wherein the processor is configuredto: determine the one or more classification factors associated with thecardiac signal by determining one or more of a ventricular pacingfactor, a noise oversensing factor, and a T-wave oversensing factor; andidentify the cardiac signal sensed during a respective one of theplurality of predetermined time intervals as the unclassified event byone or more of: determining the ventricular pacing factor indicatingventricular pacing that exceeds a ventricular pacing threshold;determining the noise oversensing factor indicating oversensing causedby noise; and determining the T-wave oversensing factor indicatingT-wave oversensing.
 14. The device of claim 9, wherein the processor isconfigured to determine the one or more classification factorsassociated with the cardiac signal by: determining sensed event windowshaving predetermined interval pairs in response to the sensed cardiacsignal during a predetermined time period; determining whether a numberof the determined sense event windows is less than an interval pairthreshold; determining whether a number of intervals occurring duringthe predetermined time period and not within the sensed event windowsthat are less than a predetermined interval threshold is greater than asensed interval threshold; and determining whether a predeterminednumber of intervals occurring during the predetermined time period andnot within the sensed event windows are less than a short interval countthreshold.
 15. A non-transitory computer-readable medium comprisinginstructions for causing one or more processors to: obtain a cardiacsignal sensed via one or more electrodes; identify an atrialtachyarrhythmia episode is occurring based on analysis of the cardiacsignal; determine, for each time interval of a plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode, one or more classification factorsassociated with the cardiac signal sensed during each of the respectiveplurality of predetermined time intervals, wherein each time interval ofthe plurality of time intervals extends for an amount of time, andwherein the amount of time corresponding to a first time interval of theplurality of time intervals is the same as the amount of timecorresponding to each other time interval of the plurality of timeintervals; identify, for each time interval of the plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode, the cardiac signal sensed during each ofthe respective plurality of predetermined time intervals as one of anatrial fibrillation (AF) event, a non-AF event, or an unclassified eventbased on the one or more determined classification factors, wherein anunclassified event represents an event which is not classified as an AFevent or a non-AF event; maintain a count of a number of time intervalsof the plurality of predetermined time intervals occurring afteridentification of the atrial tachyarrhythmia episode that are identifiedas unclassified events; determine the count exceeds an unclassifiedevent count threshold; detect termination of the atrial tachyarrhythmiaepisode in response to determining the count exceeds the unclassifiedevent count threshold; and store information related to the atrialtachyarrhythmia episode.
 16. The non-transitory computer-readable mediumof claim 15, wherein the instructions cause the one or more processorsto provide the information related to the atrial tachyarrhythmia episodeto a user.
 17. The non-transitory computer-readable medium of claim 15,wherein the atrial tachyarrhythmia episode comprises an AF episode, andwherein the instructions cause the one or more processors to: calculatean AF burden based on at least the information related to the AFepisode; determine that the AF burden exceeds a predetermined threshold;and provide an alert to a user in response to determining that the AFburden exceeds the predetermined threshold.
 18. The non-transitorycomputer-readable medium of claim 15, wherein the instructions cause theone or more processors to: identify one of the plurality ofpredetermined time intervals occurring after identification of theatrial tachyarrhythmia episode as a non-AF event; and detect terminationof the atrial tachyarrhythmia episode in response to identifying one ofthe plurality of predetermined time intervals occurring afteridentification of the atrial tachyarrhythmia episode as the non-AFevent.
 19. The non-transitory computer-readable medium of claim 15,wherein to determine the one or more classification factors associatedwith the cardiac signal, the instructions cause the one or moreprocessors to determine one or more of a ventricular pacing factor, anoise oversensing factor, and a T-wave oversensing factor; and whereinidentifying the cardiac signal sensed during a respective one of theplurality of predetermined time intervals as the unclassified event isbased on one or more of: the ventricular pacing factor indicatingventricular pacing that exceeds a ventricular pacing threshold; thenoise oversensing factor indicating oversensing caused by noise; and theT-wave oversensing factor indicating T-wave oversensing.
 20. Thenon-transitory computer-readable medium of claim 15, wherein to identifythe signal as the unclassified event based on the classificationfactors, the instructions cause the one or more processors to: determinesensed event windows having predetermined interval pairs in response tothe sensed cardiac signal during a predetermined time period; determinewhether a number of the determined sense event windows is less than aninterval pair threshold; determine whether a number of intervalsoccurring during the predetermined time period and not within the sensedevent windows that are less than a predetermined interval threshold isgreater than a sensed interval threshold; and determine whether apredetermined number of intervals occurring during the predeterminedtime period and not within the sensed event windows are less than ashort interval count threshold.